Influence of the Processes Parameters on the Properties of The ...

Chapter 2.
<strong>Processes</strong> to Manufacture Foams and to Functionalize <strong>the</strong> Surface
mechanical properties matching those <strong>of</strong> tissues at <strong>the</strong> site <strong>of</strong> implantation,
easily processable to form a variety <strong>of</strong> shapes and sizes.
In batch foaming, a polymer in disc or powder form is subjected to supercritical CO 2 flow without
mixing. After venting <strong>the</strong> CO 2 by depressurization, <strong>the</strong>rmodynamic instability causes supersaturation <strong>of</strong> <strong>the</strong>
CO 2 dissolved in <strong>the</strong> polymer matrix and hence, nucleation <strong>of</strong> cells occurs. The growth <strong>of</strong> <strong>the</strong> cells continues
until <strong>the</strong> polymer vitrifies. The saturation pressure, <strong>the</strong> saturation temperature and <strong>the</strong> depressurization rate
are <strong>the</strong> critical parameters in determining <strong>the</strong> number <strong>of</strong> cells and <strong>the</strong> cell size distribution. To predict <strong>the</strong>
pore size <strong>of</strong> foams created, in <strong>the</strong> literature, <strong>the</strong>re is several numbers <strong>of</strong> influential studies on diffusioninduced
pore growth. These models consider <strong>the</strong> polymer as a Maxwell fluid and solve systems <strong>of</strong> partial
differential equations by numerical methods [Goel and Beckman, 1995; Arefmanesh and Advani, 1991].
The cell number density increased and <strong>the</strong> cell size decreased with increasing pressure and
decreasing temperature. A high degree <strong>of</strong> super-saturation <strong>of</strong> dissolved CO 2 at high pressure and low
temperature are responsible for such results. Classical homogeneous nucleation <strong>the</strong>ory is generally used to
calculate <strong>the</strong> nucleation rate in foaming with supercritical CO 2 . The energy barrier for nucleation in <strong>the</strong>
<strong>the</strong>ory can be calculated as a function <strong>of</strong> <strong>the</strong> interfacial tension <strong>of</strong> <strong>the</strong> binary mixture and <strong>the</strong> magnitude <strong>of</strong>
<strong>the</strong> pressure drop. The <strong>the</strong>ory suggests that <strong>the</strong> energy barrier and <strong>the</strong> interfacial tension decrease as <strong>the</strong>
pressure drop increases. Consequently, <strong>the</strong> nucleation rate increases and a large number <strong>of</strong> small cells is
obtained. In fact, both <strong>the</strong> pressure drop rate and <strong>the</strong> magnitude <strong>of</strong> <strong>the</strong> pressure drop determine <strong>the</strong> cell
density and cell size in microcellular foaming. The higher <strong>the</strong> pressure drop rate, <strong>the</strong> greater <strong>the</strong> nucleation
rate due to <strong>the</strong> high supersaturation rate. This allows only a short time for existing cells to grow and,
consequently, is in favour <strong>of</strong> formation <strong>of</strong> small cells. Classical nucleation <strong>the</strong>ory fails to incorporate <strong>the</strong>
effect <strong>of</strong> <strong>the</strong> pressure drop rate. Moreover, a noteworthy study on CO 2 -assisted microcellular foaming <strong>of</strong>
PLGA is reported by Sparacio and Beckman [1998], in which a minimum in cell size with increasing
pressure was found instead <strong>of</strong> <strong>the</strong> levelling <strong>of</strong>f according to <strong>the</strong>ory. A plausible explanation is low resistance
to cell growth due to a large decrease in <strong>the</strong> melting point <strong>of</strong> <strong>the</strong> polymer and very low interfacial tension at
high pressure.
Detailed studies <strong>of</strong> <strong>the</strong> glassy polymer – CO 2 system by Wessling et al. [1994] suggest that <strong>the</strong>
nucleation mechanism underlying <strong>the</strong> foaming process is heterogeneous in nature. The significant advance
made by [Wessling et al., 1994] was that <strong>the</strong>y were able to detect and explained <strong>the</strong> appearance not only <strong>of</strong>
<strong>the</strong> porous structure in <strong>the</strong> polymer film after saturation with CO 2 but also <strong>of</strong> a dense layer next to <strong>the</strong> porous
layer. They provided a physical explanation and a ma<strong>the</strong>matical model to predict <strong>the</strong> thickness <strong>of</strong> this dense
layer. The studies <strong>of</strong> McCarthy and coworkers [Stafford et al., 1999; Arora et al., 1998a] on <strong>the</strong> effect <strong>of</strong> <strong>the</strong>
residual oligomer in polystyrene on its foaming with scCO 2 have shown that its presence affects <strong>the</strong> cell size
in <strong>the</strong>se foams. This work also questioned <strong>the</strong> ability <strong>of</strong> classical nucleation <strong>the</strong>ory to explain <strong>the</strong> foaming
mechanism in <strong>the</strong>se systems, and <strong>the</strong> authors suggest a spinodal mechanism as an alternative route <strong>of</strong> cell
formation [Stafford et al., 1999]. Foaming <strong>of</strong> polypropylene has also been studied extensively by Park and
Cheung [1997], with <strong>the</strong> most recent report by Liang and Wang [1999], who highlighted <strong>the</strong> effect <strong>of</strong>
temperature drop during depressurization <strong>of</strong> <strong>the</strong> polymer in equilibrium with high-pressure CO 2 . Handa and
Zhang [2000] used <strong>the</strong> existence <strong>of</strong> a rubbery state in <strong>the</strong> PMMA at low temperatures to generate foams by
saturating <strong>the</strong> polymer with CO 2 at 24C to 90C. They demonstrated that <strong>the</strong> solubility <strong>of</strong> CO 2 in <strong>the</strong>
polymer plays an important role in controlling cell density and cell size. Thus, <strong>the</strong> solubility <strong>of</strong> CO 2 at 34
bars and temperature <strong>of</strong> – 0.2C is 22.5% (w/w), while at <strong>the</strong> same pressure but at 24C, <strong>the</strong> solubility is just
7.9% (w/w).
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